1. Field of the invention
The present invention relates to an automatic circuit switching device for data communication, particularly to an automatic circuit switching device which switches over from a private circuit serving as a regular circuit to a public data exchange circuit, such as an Integrated Services Digital Network (ISDN) network circuit, arranged to be connected as a stand-by circuit, in the case of deterioration of the quality and disconnection of the regular circuit.
2. Description of Related Art
FIG. 1 is a block diagram showing an automatic circuit switching device of the conventional type.
The automatic circuit switching device of the conventional type usually accommodates private circuit 2 used as regular circuit through modem 23.sub.1, 23.sub.2 and an ISDN network circuit prepared for use as stand-by circuit 4 to back up the operation through corresponding terminal adaptor circuits 25.sub.1, 25.sub.2 at the legs of switching units 22.sub.1, 22.sub.2 which are connected to the each subscriber terminal 21.sub.1, 24.sub.2.
In FIG. 1, when the regular circuit between modems 23.sub.1 and 23.sub.2 is disconnected due to some trouble, data transmission between subscriber terminals 21 on the side of the host machine and subscriber terminal device 24 is stopped, thereby abnormality is usually detected at subscriber terminal 21 through monitoring of communications protocol. After abnormality is detected, a maintainer of the system operates circuit switching units 22.sub.1 and 22.sub.2 manually to connect a data communication bus to stand-by circuit 4 to back up the system.
In order to switch the circuit automatically, output signals of carrier detecting circuits provided respectively in modems 23.sub.1 and 23.sub.2 are used. Since the output signal of this carrier detecting circuit turns to "ON" when carrier is detected on regular circuit 2 and turns to "OFF" when the carrier is disconnected, modems 23.sub.1, 23.sub.2 detect the disconnection of the carrier only when the regular circuit is disconnected due to some trouble by keeping carrier constantly in the sending state.
Therefore, the automatic circuit switching is conducted by operating circuit switching units 22.sub.1, 22.sub.2 when the disconnection of carrier is detected using the output signal of the carrier detecting circuit as the control signal.
When regular circuit 2 is restored, the carrier is also sent again and the output of the carrier detecting circuit turns "ON", thereby it is possible to switch back both circuit switching units 22.sub.1 and 22.sub.2 automatically through the output signal of the carrier detecting circuit as the control signal.
When there are identification bits which can show presence of transmitted data in a data transmission units in use, the automatic switching of the circuit can be performed in the same way by using the data transmission units instead of modems 23.sub.1 and 23.sub.2.
However, the automatic circuit switching device of the conventional type fails in the normal connection of data (bit irregularity). In the worst case, restarting of the host and subscribers terminal is required when it is switched back from the stand-by circuit to the regular circuit either manually or automatically, because the regular circuit and stand-by circuit are not typically synchronized with each other.